Originally Posted by
Seiran
What I don't understand is how the turbofan can accelerate the air faster?
Let's start at the inlet. If you look closely at any turbofan engine mounted in a nacelle (such as on airliners and business jets) you will see that the inlet area is smaller than the fan area. The inlet duct is divergent, so the air slows down before it meets the fan. A careful combination of fan diameter, blade shape, fan RPM and duct divergence ensures the air passing the tips of the fan blades remains subsonic. The fan and following stator stage then increases the pressure of the air but leaves the velocity largely unchanged. This excess pressure is then converted to velocity in the bypass duct, which is convergent. I suggest you might wish to read up on Bernoulli's theorem, divergent and convergent ducts, and Momentum Theory, which should be covered in most fluid dynamics text books. Wikipedia has some good articles on these topics also.
On your questions regarding turboprops and prop fans, a turboprop has to accept air at the airspeed the aircraft is flying; it does not have the benefit of a divergent inlet duct to slow the air down. It thus has a lower outlet velocity also, but for a given diameter has a higher mass flow and thus better propulsive efficiency at lower speeds. Turboprops also lose energy to causing the air mass to rotate (whirl) as it leaves the propeller disc, something turbofans do not suffer from because stator blades behind the fan convert the whirl into additional rearward velocity.
Prop fans are somewhere in between turbo props and turbo fans. They are normally 2-stage contra-rotating to minimise whirl losses, have complex blade shapes to reduce tip losses and generally have many blades with a low aspect ratio, permitting high disc loading and high RPM while keeping the tips subsonic. This provides most of the benefits of a turbofan in terms of having a higher exit velocity than s propeller, but without a big heavy and draggy duct that turbofans have to have. The downside is the noise!
Just one last point: the simplified thrust equation. Thrust = air mass flow x velocity difference across the rotor disc (Vout - Vin), but propulsive efficiency is all about outlet velocity. For a given thrust you can either have a small mass flow and a massive velocity difference, or you can have a huge mass flow and a small velocity difference. The former case is for a pure turbojet, the latter for a turboprop. The former is great for high flight speed but has low propulsive efficiency; the latter has lower flight speed but good propulsive efficiency. The key point here is that the outlet velocity sets the maximum level flight speed of the aircraft, because to create thrust the outlet velocity must exceed the aircraft flight speed. However for propulsive efficiency the velocity difference should be small and the mass flow large, hence the trend for larger diameter turbofan engines over the last few decades.
Regards, WF